The bleaching of wood pulp to obtain lighter grades of pulp to produce correspondingly lighter or whiter grades of paper finds its origin long into the past as indicated by U.S. Pat. No. 11,343 (July 18, 1854).
Since this time many advances have been made in the processes, however modern methods still utilize chlorination, caustic extraction to dissolve chlorinated lignins, and final hypochlorite bleaching in one or more stages.
The theory of reactions occurring in chlorination and hypochlorite bleaching of alkaline pulps for example is described quite comprehensively in Chapters II and IV of "The Bleaching of Pulp" Tappi Monograph Series No-10. Basically calcium and sodium hypochlorite solutions have been utilized for the bleaching of pulp which for the most part require alkaline solutions.
As is well-known, different grades of paper require different degrees of brightness of the pulp. In order to obtain the brightness level desired, the pulp has a certain demand of calcium or sodium hypochlorite (commonly referred to as "hypo") to perform the function desired to in turn produce the desired brightness level. These parameters are well defined by the mill personnel only because of comprehensive testing and experience. Accordingly for a desired brightness for a certain pulp, mill personnel can quite accurately estimate the conditions of bleaching required together with the hypo demand. The overall costs of the bleaching operation even if only the costs of the bleaching chemicals are considered, is quite substantial. Accordingly mills look favorably upon any additives or procedural innovations which can lessen these costs to any degree.
It was to this objective that the present inventors directed their attention. The present inventors felt that if the bleaching reaction could be controlled relative to the rate of reaction that perhaps greater bleaching efficiency could be obtained. It was discovered that if a low molecular weight water soluble polymer of acrylic acid (or water soluble salt) having a molecular weight of approximately 500 to 20,000 was added to the hypo solution, that the rate of reaction in fact was controlled so as to provide what was believed to be a slower bleaching cycle, or slower reaction rate thereby a smoother, more even, and accordingly a more effective bleaching operation. Because of these achievements, less hypo was required to obtain a prescribed brightness value. This represented a cost savings which obviously was quite impressive to bleaching operations' personnel. Treatments using from about 0.5 to 100, and preferably from 1.0 to 50 parts per million parts of pulp slurry were found to be quite effective. Although the polymers described have been found to be suitable for the purpose when used alone, the preferred treatment is a combination of the polymer with carboxymethyl cellulose (or water soluble salt thereof and preferably the sodium salt) in a weight ratio of from 12:1 to 1:12 polymer to methylcellulose with the preferable ratio being 10:1 to 1:6. The desired treatment range with the combination was also 0.5 to 100 ppm of pulp slurry and preferably 1.0 to 50 ppm.